Lubricating greases contain three primary components: base oil, thickener and additives. The base oil provides lubricity, the thickener gives body and structure to the grease, and the additives increase resistance of the grease to oxidation, rust, corrosion and the like.
Conventionally base oils used in greases include mineral and synthetic oils and mixtures of each. Examples of mineral oils used are paraffinic and naphthenic oils. Examples of synthetic oils used are: esters including but not limited to polyol and dibasic acid esters, poly-glycols, synthetic hydrocarbons such as PAO (poly alpha olefins) and silicone oils.
A wide variety of materials are used as thickeners in lubricating greases, the selection of which often depends upon the application for the grease. Among the thickeners used in lubricating greases are intended soaps and complex soaps of these metals: aluminum, barium, calcium, iron, lead, lithium, magnesium, potassium, sodium, strontium, tin, titanium, zinc, or the following non-soap thickeners: calcium sulfonate, clay, pigments, polyurea, and polytetrafluoroethylene.
Additives that are used in greases include antioxidants, anticorrodants, metal deactivators, colorants and the like.
Greases are formulated with the view of meeting performance criteria based upon the intended use. For example, lubricating greases used in aviation applications are often required to have specific performance properties at both low and high temperatures. Among these properties are low temperature torque and high temperature evaporation. Low temperature torque is a measurement used to determine whether a grease is too firm such as to inhibit bearings from rotating freely at low temperatures. Good low temperature performance can be achieved by using an oil in the grease that is low in viscosity. The low temperature properties of a grease are typically measured using the ASTM method D 1478, Low Temperature Torque Of Ball Bearing Greases, which measures the freedom with which a ball bearing packed with grease operates when cooled to a specified temperature. The freedom of bearing rotation is reported by the D 1478 method, as the torque required to start and maintain rotation of the bearing. Torque is typically reported in units of Newton-meters.
One of the high temperature properties of a lubricating grease important to aviation applications is low evaporation. If the oil evaporates it is not available to lubricate and the performance of the grease lubricated component will be jeopardized. This is the case when aircraft lubricants are exposed to high temperatures as well as reduced atmospheric pressure when operating during flight. Both conditions can promote the evaporation of the oil used in the grease. An approach for avoiding excessive evaporation is to use greases with high base oil viscosity. Tests typically used to measure the evaporation loss of lubricating grease are the ASTM test methods D 972, and D 2595, Evaporation Loss Of Lubricating Greases. In each of these tests heated air passes over the surface of the grease for a specified time with the air heated to a specified temperature as called for by the aviation grease specification.
One particular specification, Boeing Material Specification, BMS 3-33A, Grease, Aircraft, General Purpose, calls for a grease in which the low temperature starting torque in the ASTM D 1478 test shall not exceed 0.10 Newton-meter and in which the evaporation weight loss of the grease, when evaluated in the ASTM D 2595 test for 500 hours at 121° C. shall not exceed 10.0 wt %.
When the type of oil selected for a grease to meet the Boeing Material Specification, BMS 3-33A is 100% poly alpha olefin (PAO), experience has shown that difficulty is encountered because the low viscosity PAO needed to meet the low temperature torque requirements results in excessive high temperature evaporation. One approach to overcome this difficulty is to use PAO blends with synthetic esters as the grease base oil; however, esters may hydrolyze in hot and wet environments and produce species that may be corrosive to various metals. Therefore it would be desirable to provide a grease having a PAO base oil that is free of polyesters and that can meet the Boeing specification.